Image processing techniques for correcting distortion of an image taken by an electronic camera or the like are known (refer to Patent Document 1 and Non-patent Document 1, for example). In such distortion correction, it is necessary that a distortion amount distribution in an image be known.
A distortion amount D that is caused in an image by an imaging lens is given, where Y is the image height of an ideal image point and Y0 is the image height of an actual image point, by the following equation:D=100×(Y−Y0)/Y0(%)
The distribution of the distortion amount D in an image is a function of the image height ratio r (=(image height)/(maximum image height)) and can be approximated as follows:D(r)≡a1r+a2r2+a3r3+a4r4+a5r5+a6r6+ . . .
In many cases, a good enough approximation can be obtained even if the order is reduced to 3 in the following manner:D(r)≡a1r+a2r2+a3r3 
The coefficients (hereinafter referred to as distortion coefficients) a1, a2, and a3 of this function which represents the distortion amount distribution vary depending on not only the lens type (specification) but also the lens position (combination of the focal length f and the focal position d) at the time of shooting.
For the above reason, in the camera disclosed in Patent Document 1, a table containing distortion coefficients a1, a2, and a3 for each lens position is prepared in advance as the distortion correcting information. In taking an image, the camera can easily calculate a distortion amount distribution in the image by referring to this table based on a lens position.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2003-110847    Non-patent Document 1: Yuto Takahashi, “Lens Designing,” Tokai University Press, p. 127, Mar. 20, 1994.